The present invention relates to a grinder and a grinding method for significantly precisely grinding at least one sides of a hard and thin work, for example a wafer utilized for a semiconductor device.
In general, a conventional grinder has spindles rotatively supported by spindle heads thereof in such a manner that a grinding wheel is secured to the leading end of each spindle. Moreover, a feeding unit comprising a motor and a ball screw is connected to the spindle head. When the spindle heads are fed and moved in the axial direction by the feeding units while the grinding wheels are rotated by the rotating motors, the outer surfaces of the work are ground.
The conventional grinder has the structure that each unit for feeding the grinding wheel comprises the motor and the ball screw. When the ball screw is rotated by the motor, the operation for feeding the spindle head is performed. However, the grinding wheels cannot precisely be fed because of insufficient rigidity of the machine and frictional resistance of sliding portions when the feeding operation must precisely be performed in order of microns or sub-microns. Thus, there arises a problem in that a precise grinding operation cannot be performed.
As a grinder capable of grinding both side surfaces of the work, Japanese Utility-Model Examined Publication No. Hei. 1-8282 teaches a conventional twin-head grinder, for example. The grinder disclosed therein has a structure that a C-shape column frame allowed to project over the frame of the grinder by a cantilever method forms a grinding head for supporting the upper grinding wheel. A bending moment acts on the column by a reaction of the grinding operation which acts on the grinding wheel because of a grinding resistance generated during the machining operation. Moreover, a thermal displacement takes place, causing a precise grinding operation to be inhibited. Therefore, a frame having an upper bed disposed above a lower bed has been disclosed. Moreover, the upper and lower grinding wheels provided for the foregoing frame are mounted to upper and lower spindles. The spindles are rotated by motors and belts disposed on the sides of the spindles. The upper spindle is vertically moved by a feeding means comprising a rack which is operated by hydraulic pressure or air pressure.
As a means for vertically moving the upper grinding wheel, a structure has been disclosed in, for example, Japanese Patent Unexamined Publication No. Sho. 61-270043. The means comprises a feeding means for vertically moving spindle heads by motors, ball screws and nuts provided for the spindle heads. The main spindle is, in the spindle head, supported by a hydrostatic bearing. Moreover, each grinding wheel is rotated by a built-in type motor.
The conventional grinder as disclosed in the above-mentioned Japanese Patent Unexamined Publication No. Sho. 61-270043 has realized a frame which is free from a thermal displacement and a rotatively supporting means with which the main spindles are not thermally expanded. On the other hand, this conventional grinder has the feeding mechanism comprising the rack or the ball screw which is operated by the motor serving as the drive source and having the structure that the grinding heads are vertically guided along the sliding surfaces, so that there is a problem that the conventional grinder cannot substantially perform precise feeding because of a sliding resistance or the like.
Further, in the conventional grinder, the motors which are drive sources for vertically moving the grinding heads and the ball screws are disposed on the sides of the spindles for rotating the grinding wheels, so that a lateral load is applied to the spindle. Accordingly, there is a great possibility that smooth and precise feeding cannot be performed and intercepted.